Use git ls-tree to collect source files instead of walking the fs

test/unittests/BUILD.gn

 # Copyright 2016 The V8 project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("../../gni/v8.gni")
 
 v8_executable("unittests") {
   testonly = true
 
   sources = [
+    "../../testing/gmock-support.h",
+    "../../testing/gtest-support.h",
     "base/atomic-utils-unittest.cc",
     "base/bits-unittest.cc",
     "base/cpu-unittest.cc",
     "base/division-by-constant-unittest.cc",
     "base/flags-unittest.cc",
     "base/functional-unittest.cc",
     "base/ieee754-unittest.cc",
     "base/iterator-unittest.cc",
     "base/logging-unittest.cc",
     "base/platform/condition-variable-unittest.cc",
@@ skipping 159 matching lines @@
 
     # Suppress warnings about importing locally defined symbols.
     if (is_component_build) {
       ldflags = [
         "/ignore:4049",
         "/ignore:4217",
       ]
     }
   }
 }